The molten slag produced in the smelting of iron ore can be transformed into a solid granulate in the so-called jet process by directing a plurality of fine water sprays at a stream of the molten slag to break it up into slag particles. These quenched particles are caught in a basin or pit filled with water where they are completely quenched. Thereafter the water is drained away from the slag and the finished product is used as is.
Such slag has several serious disadvantages. First of all the particles pick up a considerable percentage, often between 10% and 20% by weight, of water. This extra water increases the transport costs for the slag or necessitates a later drying operation. Furthermore the particles thus produced are extremely compact and glass-like so that milling them or grinding them to a smaller size is a burdensome process, indeed in the cement industry up to 20% of the energy costs are often expended for such grinding. Obviously when between 70% and 75% of the slag cements are made from blast-furnace slag a considerable saving would result from any reduction in the moisture content of the granules.
Slag sand is also produced as an undesired secondary product in the production of expanded or lightweight blast-furnace slag. Such expanded slag is normally produced by flowing and mixing the stream of slag directly with water so as to expand it. The still pyroplastic mass is then poured over a rotating drum that subdivides the stream into tiny particles which are thrown through the air or through a water-mist cloud for sufficient cooling that the particles, when they eventually come to rest, are no longer plastic. A very small quantity of water, in the neightborhood of 0.7 m.sup.3 -1.0 m.sup.3 per ton of slag, is normally used to produce the desired relatively large clinker.
During the production of expanded slag as described above the fines under a mesh size of 3 mm are then screened out. These fines constitute slag sand, and rarely constitute more than 25% by weight of the expanded slag. For this reason the slag-expanding processes are normally set up to minimize the production of such slag sand whose separation is more trouble than it is generally considered to be worth.